Mobile ad-hoc networks are characterized by a capability for organizing themselves, without predefined infrastructure, to ensure the transport of communications with a specified quality and for configuring themselves automatically so as to respond to the system deployment requirements. Moreover such a network must be able to ensure its maintenance in an autonomous manner, in particular the nodes of the network being mobile, it must be possible for the topology of the network to be updated continually.
In a mobile ad-hoc network, it is indispensable to design and implement a process for simultaneous access to the transmission resources so as to prevent simultaneous communications between several users or groups of users from interfering with one another.
The problem of the removal or limitation of interference between users in a mobile network is crucial since it directly impacts the service quality that can be provided by such a network.
The problematic issue of interference between users in a wireless network is treated in diverse ways in the state of the art.
Firstly, solutions are known which are based on the application of specific transport protocols with built-in processes for monitoring and correcting interference related errors at the price of a decrease in the useful throughput. For example, processes of ARQ “Automatic Repeat Request”, FEC “Forward Error Correcting” or H-ARQ “Hybrid Automatic Repeat Request” type are used to combat the disturbances engendered by the interference of communications between users but they introduce either redundancy data which impact the useful throughput or an additional transmission lag that is sometimes incompatible with certain real-time applications such as voice over IP. By way of example, document [1] presents a study of the transport protocols making it possible to combat the phenomenon of interference in a wireless network.
Other known processes are based on the use of routing protocols, implemented at the network layer level, with the aim of maximizing the utilization of the resources of the wireless network. By way of example, the process described in document [2] is based on estimation of the interference generated by the communications, that described in document [3] takes into account an anticipation of future interference.
Finally there exist processes for simultaneous access to the physical resources of the transmission medium which are implemented at the level of the physical layer or of the MAC layer. These processes are aimed at sharing the resources between the various users so as to avoid collisions.
The CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) multiple access process uses a mechanism for dodging collisions between communications which is based on a principle of reciprocal acknowledgement of receipt between the sender and the receiver. If the network is congested, transmission is deferred. In the converse case, if the transmission medium is available for a given time, transmission is carried out for this time. The sender transmits a message containing information about the volume of the data that it desires to send and its transmission speed. The receiver dispatches a message to the sender telling it that the medium is available for a send, and then the sender begins sending its data. After reception of all the data sent, the receiver dispatches an acknowledgement of receipt to the sender. All the neighbouring senders then wait a time estimated to be necessary for the transmission of the volume of information to be sent. The CSMA/CA access process is notably used by Wifi networks.
The CDMA (Code division multiple access) multiple access process relates to a system for coding transmissions which is based on the spread spectrum technique. Several digital links can use the same carrier frequency by virtue of a spreading code allocated to each. The receiver uses the same spreading code to demodulate the signal that it receives and to extract the useful information. The spreading codes used have an orthogonality property which guarantees the avoidance of collisions between communications of various users. The code itself does not transport any useful information. The despreading operation on reception requires significant calculation capabilities, therefore more expensive components.
The FDMA (Frequency division multiple access) multiple access process consists in slicing the useful frequency band into sub-bands so as to allot a part of the spectrum to each user. In this way, the collisions between users which do not communicate on the same frequencies, are reduced.
Finally, TDMA (Time division multiple access) multiple access processes are known, based on a temporal division of the resources and of the multiple access to the transmission medium. These processes implement a time slicing into disjoint windows 100 each lasting a duration equal to a predetermined period T. Each time window 100 is divided into a plurality S of time slices 110 of duration T/S also called slots 110. An exemplary time slicing into windows 100 and slots 110 is represented in FIG. 1. When one terminal wishes to communicate with another, they agree over the choice of the time slots 110 during which the communication will be carried out by taking into account the communications performed by the other terminals in the vicinity so as to avoid all collisions with them. This process guarantees service quality but requires the mutual synchronization of all the terminals of the network as well as a knowledge of the communications undertaken by the other terminals so as to make at any instant the right choice of allotment of time slots to each user. In order to operate, this process therefore requires cooperation between the terminals and this may pose a problem for networks of large size.
To achieve this cooperation on a meshed network whatever its size, one process consists in introducing a hierarchy between the terminals. The terminals are thus grouped into subsets, also called clusters, in which a unique terminal, called the cluster-head, acts as leader and has the function of scheduling the communications in the slots of the TDMA time window for all the terminals of the cluster while taking care to see that there are no collisions between the communications of each terminal. The other terminals of the cluster have a direct tie with the cluster-head, they are either members if they are linked only to members of the cluster, or relay nodes if they have a tie with a terminal belonging to another cluster.
An exemplary meshed network topology without particular hierarchy is represented in FIG. 2. The same network organized into clusters is represented in FIG. 3. Each cluster is identified by its cluster-head terminal, referenced by the letter C and a number, and by a circle of radius substantially equal to the range of a radio link between two terminals. The terminals situated inside a circle belong to cluster i managed by cluster-head terminal C.
Such an organization into clusters makes it possible to delegate the allocation of the resources to certain terminals for a group of adjacent terminals. Each cluster manages its communications independently of the other clusters, thereby allowing management of the resources on large-scale networks.
However, this independent management by clusters entails risks of collisions between the communications of two adjacent clusters. An adjacent cluster is a cluster immediately neighbouring another. In the example of FIG. 3, the clusters C2, C3 and C5 are adjacent to the cluster C1. Likewise the clusters C4, C5 and C7 are adjacent to the cluster C6. The clusters having no knowledge of the slots chosen by the other clusters, it is possible that two clusters may be sufficiently close for the communications of one to impede the communications of the other and may choose the same slots for their communications, thus creating collisions between their communications.
A problem with the TDMA multiple access processes associated with an organization of the meshed network into clusters is therefore that the prevention of collisions or interference between clusters is not guaranteed. For it to be so, the process for accessing the resources must make it possible to minimize the chances of two or more adjacent clusters choosing the same time slots to communicate.
A solution to this problem is presented in document [4]. It is based on the definition of super time windows composed of a plurality Nf of TDMA time windows. Each super window is assigned to a different cluster, in an arbitrary manner, with the sole constraint of not assigning the same super window to two adjacent clusters. This solution makes it possible to avoid interference between neighbouring clusters, however, its effectiveness is limited since it entails a lag equal to at least the duration of a super window, between two communications. Moreover the number Nf of time windows allocated to a cluster is defined as a function of an a priori estimation of the number of neighbouring or adjacent clusters. Thus, this process does not take into account the dynamic aspect of the topology of a mobile network and does not allow optimal apportioning of the resources for clusters which exhibit a number less than Nf of adjacent clusters.